Phase change memory structure and the same

ABSTRACT

The present disclosure provides a phase change memory structure, including a bottom electrode, a first phase change material contacting a top surface of the bottom electrode, a first switch over the first phase change material, a second phase change material over the first switch, and a top electrode over the second phase change material.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/964,900, filed Apr. 27, 2018, and claims the benefit thereof under 35U.S.C. 120.

BACKGROUND

Phase change technology is promising for next generation memories. Ituses chalcogenide semiconductors for storing states. The chalcogenidesemiconductors, also called phase change materials, have a crystallinestate and an amorphous state. In the crystalline state, the phase changematerials have a low resistivity, while in the amorphous state they havea high resistivity. The resistivity ratios of the phase change materialsin the amorphous and crystalline states are typically greater than 1000and thus the phase change memory devices are unlikely to have erroneousreading. The chalcogenide materials are stable at certain temperatureranges in both crystalline and amorphous states and can be switched backand forth between the two states by electric pulses. One type of memorydevice that uses the principal of phase change in chalcogenidesemiconductors is commonly referred to as phase change random accessmemory (PCRAM).

PCRAM has several operating and engineering advantages, including highspeed, low power, non-volatility, high density, and low cost. Forexample, PCRAM devices are non-volatile and may be written into rapidly,for example, within less than about 50 nanoseconds. The PCRAM cells mayhave a high density. In addition, PCRAM memory cells are compatible withCMOS logic and can generally be produced at a low cost compared to othertypes of memory cells.

Phase change material is formed between top electrode and bottomelectrode contact. In a reset operation, phase change material may beheated up to a temperature higher than the melting temperature when acurrent passes through it. The temperature is then quickly dropped belowthe crystallization temperature. A portion of the phase change materialis changed to an amorphous state with a high resistivity, thus the stateof the PCRAM cell is changed to a high-resistance state. Region can beset back to the crystalline state by heating up the phase changematerial to a temperature higher than the crystallization temperature,but below the melting temperature, for a certain period.

Conventional phase change memory structure includes a phase changematerial, an ovonic threshold switch, a middle electrode, a topelectrode, and a bottom electrode between adjacent oxide layers.However, in order to combine material properties of a plurality of phasechange materials or ovonic threshold switches, one would combine severalphase change structure, that is, the plurality of phase change materialsor ovonic threshold switches would be disposed at different layers, withoxide layers spacing in-between. Thus the thickness of the whole phasechange structure became thicker.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a cross section of a semiconductor structure, in accordancewith some embodiments of the present disclosure.

FIG. 2A is a cross section of a phase change memory structure, inaccordance with some embodiments of the present disclosure.

FIG. 2B is a cross section of a phase change memory structure, inaccordance with some embodiments of the present disclosure.

FIG. 2C is a cross section of a phase change memory structure, inaccordance with some embodiments of the present disclosure.

FIG. 3A is a cross section of a phase change memory structure, inaccordance with some embodiments of the present disclosure.

FIG. 3B is a cross section of a phase change memory structure, inaccordance with some embodiments of the present disclosure.

FIG. 3C is a cross section of a phase change memory structure, inaccordance with some embodiments of the present disclosure.

FIG. 4A is a cross section of a phase change memory structure, inaccordance with some embodiments of the present disclosure.

FIG. 4B is a cross section of a phase change memory structure, inaccordance with some embodiments of the present disclosure.

FIG. 4C is a cross section of a phase change memory structure, inaccordance with some embodiments of the present disclosure.

FIG. 5A is a cross section of a phase change memory structure, inaccordance with some embodiments of the present disclosure.

FIG. 5B is a cross section of a phase change memory structure, inaccordance with some embodiments of the present disclosure.

FIG. 5C is a cross section of a phase change memory structure, inaccordance with some embodiments of the present disclosure.

FIG. 6A is a cross section of a phase change memory structure, inaccordance with some embodiments of the present disclosure.

FIG. 6B is a cross section of a phase change memory structure, inaccordance with some embodiments of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the disclosure are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contains certainerrors necessarily resulting from the standard deviation found in therespective testing measurements. Also, as used herein, the term “about”generally means within 10%, 5%, 1%, or 0.5% of a given value or range.Alternatively, the term “about” means within an acceptable standarderror of the mean when considered by one of ordinary skill in the art.Other than in the operating/working examples, or unless otherwiseexpressly specified, all of the numerical ranges, amounts, values andpercentages such as those for quantities of materials, durations oftimes, temperatures, operating conditions, ratios of amounts, and thelikes thereof disclosed herein should be understood as modified in allinstances by the term “about.” Accordingly, unless indicated to thecontrary, the numerical parameters set forth in the present disclosureand attached claims are approximations that can vary as desired. At thevery least, each numerical parameter should at least be construed inlight of the number of reported significant digits and by applyingordinary rounding techniques. Ranges can be expressed herein as from oneendpoint to another endpoint or between two endpoints. All rangesdisclosed herein are inclusive of the endpoints, unless specifiedotherwise.

Conventionally, only one switch and one phase change memory material aredisposed in one phase change memory cell. Along the path of thresholdvoltage reduction, thickness of the switch has to be made thinner to anextent that triggers substantial leakage current. Therefore, directlyreducing the thickness of the switch to lower the threshold voltage isat the expense of increasing the leakage current. In addition, havingonly one phase change material in one phase change memory cell faceshigher threat for memory cell failure due to the fact that retentionlost is a random process. For instance, if the only phase changematerial fails during operation, the entire memory cell is counted as afailed cell.

Present disclosure provides a phase change memory cell structure havinga versatile threshold voltage tuning design and a low memory cellfailure rate. By having at least two switches arranged in series in onephase change memory cell, the properties of the phase change memorystructure, such as leakage current or threshold voltage, can be easilytuned by tailoring various kinds of materials and thicknesses ofswitches. Meanwhile by having two phase change materials arranged inseries could greatly reduce failure rate of the phase change memorystructure.

Referring to FIG. 1, FIG. 1 is a cross section of a semiconductorstructure 100, in accordance with some embodiments of the presentdisclosure. The semiconductor structure 100 includes at least one phasechange memory stack 140. In some embodiments, the phase change memorystack 140 includes a top electrode 104 at a top portion of the phasechange memory stack 140, and a bottom electrode 101 at a bottom portionof the phase change memory stack 140. The phase change memory stack 140may be surrounded by a dielectric layer 127. The phase change memorystack 140 is disposed between an N^(th) inter layer dielectric(hereinafter N^(th) ILD) 128 and an (N+1)^(th) inter layer dielectric(hereinafter (N+1)^(th) ILD) 128′. Herein N is an integer greater thanor equal to 1. In some embodiments, a top surface of the top electrode104 contacts with the (N+1)^(th) ILD 128′. In some embodiments, a bottomsurface of the bottom electrode 101 contacts with the N^(th) ILD 128. Insome embodiments, the N^(th) ILD 128 and the (N+1)^(th) ILD 128′ may beformed from a variety of dielectric materials and may, for example,include oxide, nitride, silicide, carbide, metal, or the like. In someembodiments, the N^(th) ILD 128 and the (N+1)^(th) ILD 128′ may includemetal layers surrounded by the aforesaid dielectric materials. The metallayer referred herein includes metal lines and metal vias composed ofcopper or copper alloy. Metal lines and metal vias in different metallayers form an interconnect structure composed of substantially purecopper (for example, with a weight percentage of copper being greaterthan about 90 percent, or greater than about 95 percent) or copperalloys, and may be formed using the single and/or dual damasceneprocesses. Metal lines and metal vias may be, or may not be,substantially free from aluminum. Interconnect structure includes aplurality of metal layers. In some embodiments, one or more metal linesin the (N+1)^(th) ILD 128′ may be connected to the top electrode 104. Insome embodiments, one or more metal lines in the N^(th) ILD 128 may beconnected to the bottom electrode 101.

Referring to FIG. 2A, FIG. 2A is a cross section of a phase changememory structure. In some embodiments, the phase change memory structuremay include the phase change memory stack 140 disposed between theN^(th) ILD 128 and the (N+1)^(th) ILD 128′. The phase change memorystack 140 is surrounded by the dielectric layer 127. The phase changememory stack 140 includes the bottom electrode 101 disposed above theN^(th) ILD 128, a first phase change material 211 contacting a topsurface of the bottom electrode 101, a first switch 111 disposed above atop surface of the phase change material 211, a second phase changematerial 211′ disposed above a top surface of the first switch 111, andthe top electrode 104 disposed between a top surface of the second phasechange material 211′ and the bottom surface of the (N+1)^(th) ILD 128′.

In some embodiments, the first switch 111 has a high resistance undervoltages below a threshold voltage value of the first switch 111. If anapplied voltage exceeds the threshold voltage value of the first switch111, the resistance of the first switch 111 becomes significantly lower.In some embodiments, a thickness t111 of the first switch 111 is in arange of from 20 Angstrom to 500 Angstrom. If the thickness t111 of thefirst switch 111 is thinner than 20 Angstrom, film thickness uniformityof the manufactured switches may be difficult to control duringdeposition. If the thickness t111 of the first switch 111 is thickerthan 500 Angstrom, subsequent operations are likely to face high aspectratio problems, for example, while forming a through-layer via.

In some embodiments, the first switch 111 may include chalcogenidematerials including, but not limited to, one or more of As, Ge, and Se,e.g., which may be AsGeSe, N doped AsGeSe, Si doped AsGeSe, InAsGeSe, orstoichiometric materials. In some embodiments, the first switch 111 mayinclude superlattice structure, which will be subsequently described inFIG. 6A of present disclosure. In some embodiments, a thickness of thebottom electrode 101 or the top electrode 104 is in a range of from 20Angstrom to 500 Angstrom. If the thickness t101 or t104 is thinner than20 Angstrom, film thickness uniformity of the manufactured bottomelectrode 101 may be difficult to control during metal formation. If thethickness t101 or t104 is thicker than 500 Angstrom, subsequentoperations are likely to face high aspect ratio problems, for example,while forming a through-layer via.

As demonstrated in FIG. 2A, having two phase change materials arrangedin series could greatly reduce failure rate of the phase change memorystructure. For instance, if the first phase change material 211 failsduring operation, the second phase change material 211′, which arrangedin series with the first phase change material 211, could still functionnormally so that the entire memory cell, mainly composed of phase changememory stack 140, is not counted a failed cell. Still referring to FIG.2A, both the materials of the first phase change material 211 and thesecond phase change material 211′ have two distinct metastable phases,e.g. crystalline and amorphous, related to different resistivities. Insome embodiments, the first phase change material 211 and the secondphase change material 211′ may include commonly used chalcogenidematerials including, but not limited to, one or more of Ge, Te, and Sb,e.g., which may be GeSbTe, N doped GeSbTe, Si doped GeSbTe, InGeSbTe, orstoichiometric materials. In some embodiments, the first phase changematerial 211 may include superlattice structure. In some embodiments,the second phase change material 211′ may include superlatticestructure. In some embodiments, the first phase change material 211 hasthe same material with the second phase change material 211′. In someother embodiments, the first phase change material 211 has differentmaterial with the second phase change material 211′. In someembodiments, a thickness t211 of the first phase change material 211 isin a range of from 20 Angstrom to 500 Angstrom. If the thickness t211 ofthe first phase change material 211 is thinner than 20 Angstrom, filmthickness uniformity of the manufactured phase change materials may bedifficult to control during deposition. If the thickness t211 of thefirst phase change material 211 is thicker than 500 Angstrom, subsequentoperations are likely to face high aspect ratio problems, for example,while forming a through-layer via. In some embodiments, the second phasechange material 211′ has a thickness in a range similar to the firstphase change material 211. In some embodiments a thickness t211′ of thesecond phase change material 211′ is different from the thickness t211of the first phase change material 211. While in some embodiments, thethickness t211 is identical with the thickness t211′.

In some embodiments, the first phase change material 211 and the secondphase change material 211′ can switch between distinct metastable phasesby altering Joule heat provided by the bottom electrode 101 and/or thetop electrode 104. In some embodiments, the top electrode 104 and/or thebottom electrode 101 is selected to possess a suitable thermalconductivity, or are designed to possess heat-retention structures, inorder to effectively achieve the phase change temperature.

In some embodiments, the phase change memory structure illustrated inFIG. 2A may further include one first middle electrode 300 between thetop electrode 104 and the bottom electrode 101, as shown in FIG. 2B.Compared to the top electrode 104 and the bottom electrode 101, thefirst middle electrode 300 is selected to have a different thermalproperty, for example, a lower heat-retention, from its top and bottomcounterparts due to the fact that one end of the middle electrode 300 isin contact with the first switch 111, which does not require elevatedtemperature as the first or the second change material 211/211′ does. Insome embodiments, the first middle electrode 300 may be composed of oneor more conductive and/or semiconductive materials such as, for example,carbon (C), carbon nitride (C_(x)N_(y)); n-doped polysilicon and p-dopedpolysilicon; metals including, Al, Cu, Ni, Cr, Co, Ru, Rh, Pd, Ag, Pt,Au, Ir, Ta, and/or W; conductive metal nitrides including TiN, TaN, WN,and/or TaCN; conductive metal silicides including tantalum silicides,tungsten silicides, nickel silicides, cobalt silicides and/or titaniumsilicides; conductive metal silicides nitrides including TiSiN and/orWSiN; conductive metal carbide nitrides including TiCN and/or WCN; andconductive metal oxides including RuO₂. Referring to FIG. 2B, a crosssection of a phase change memory structure, the first middle electrode300 may be disposed between the first switch 111 and the second phasechange material 211′. The first middle electrode 300 may also be spacingbetween other switch-phase change material interfaces, for example,between the first switch 111 and the first phase change material 211. Insome embodiments, a thickness t300 of the first middle electrode 300 isin a range of from 20 Angstrom to 500 Angstrom. The criticality of thethickness t300 of the first middle electrode 300 is similar to thethickness t101 of the bottom electrode 101, as previously discussed inFIG. 2A.

Referring to FIG. 2C, the phase change memory structure illustrated inFIG. 2A may further include multiple middle electrodes between the topelectrode 104 and the bottom electrode 101. For example, the firstmiddle electrode 300 is spacing between the first phase change material211 and the first switch 111; while a second middle electrode 300′ isdisposed between the first switch 111 and the second phase changematerial 211′. In some embodiments, a thickness t300′ of the secondmiddle electrode 300′ is in a range similar to the thickness t300 of thefirst middle electrode 300.

Referring to FIG. 3A, FIG. 3A is a cross section of a phase changememory structure. The phase change memory structure may include thephase change memory stack 140 disposed between the N^(th) ILD 128 andthe (N+1)^(th) ILD 128′. The phase change memory stack 140 is surroundedby the dielectric layer 127. In some embodiments, the phase changememory stack 140 includes the bottom electrode 101 disposed above theN^(th) ILD 128, the first phase change material 211 contacting a topsurface of the bottom electrode 101, a first switch 111 disposed above atop surface of the first phase change material 211, a second phasechange material 211′ disposed above a top surface of the first switch111, a second switch 111′ disposed above a top surface of the secondphase change material 211′, and the top electrode 104 disposed between atop surface of the second switch 111′ and the bottom surface of the(N+1)^(th) ILD 128′.

In some embodiments, the phase change memory stack 140 may furtherinclude one or more phase change materials and/or one or more switchesbetween the top electrode 104 and the second switch 111′. In someembodiments, the first switch 111 and the second switch 111′ have highresistance under voltages below thresholds voltage value of the firstswitch 111 and the second switch 111′ respectively. If an appliedvoltage exceeds the threshold voltage value, the resistances of theswitches become significantly lower. As previously discussed in FIG. 2A,in some embodiments, a thickness t111 of the first switch 111 is in arange of from 20 Angstrom to 500 Angstrom. A thickness t111′ of thesecond switch 111′ is in a range similar to the thickness t111 of thefirst switch 111. In some embodiments the thickness t111′ of the secondswitch 111′ is different from the thickness t111 of the first switch111. While in some embodiments, the thickness t111 is identical with thethickness t111′.

As previously discussed in FIG. 2A, in some embodiments, a thicknesst101 of the bottom electrode 101 is in a range of from 20 Angstrom to500 Angstrom. A thickness t104 of the bottom electrode 104 is in a rangesimilar to the thickness t101 of the bottom electrode 101. In someembodiments, the first switch 111 and the second switch 111′ may includecommonly used chalcogenide materials including, but not limited to, oneor more of As, Ge, and Se, e.g., which may be AsGeSe, N doped AsGeSe, Sidoped AsGeSe, InAsGeSe, or stoichiometric materials. In someembodiments, the first switch 111 may include superlattice structure. Insome embodiments, the second switch 111′ may include superlatticestructure. In some embodiments, a material of first switch 111 isidentical to the second switch 111′. In some other embodiments, amaterial of the first switch 111 is different from the second switch111′. In some embodiments the thickness t111′ of the second switch 111′is different from the thickness t111 of the first switch 111. While insome embodiments, the thickness t111 is identical with the thicknesst111′.

Conventionally, only one switch is disposed in one phase change memorystructure. Along the path of threshold voltage reduction, thickness ofthe switch has to be made thinner to an extent that triggers substantialleakage current. Therefore, directly reducing the thickness of theswitch to lower the threshold voltage is at the expense of increasingthe leakage current. By having at least two switches arranged in seriesin one phase change memory structure, the properties of the phase changememory structure, such as leakage current or threshold voltage, can beeasily tuned by tailoring various kinds of materials and thicknesses ofswitches. As illustrated in FIG. 3A, the first switch 111 and the secondswitch 111′ can be composed of different materials having differentswitching properties. In some embodiments, effective threshold voltagecan be achieved by having appropriate thicknesses for the first switch111 and the second switch 111′, without sacrificing the device leakagecurrent. For example, the first switch 111 may have a greater thresholdvoltage while the second switch 111′ may have a lower threshold voltagematching with the greater threshold voltage in order to render aneffective threshold voltage desired in the phase change memorystructure.

As previously discussed in FIG. 2A, having two phase change materialsarranged in series could greatly reduce failure rate of the phase changememory structure. Referring to FIG. 3A, both the materials of the firstphase change material 211 and the second phase change material 211′ havetwo distinct metastable phases, e.g. crystalline and amorphous, relatedto different resistivity. In some embodiments, the first phase changematerial 211 and the second phase change material 211′ may includecommonly used chalcogenide materials including, but not limited to, oneor more of Ge. Te, and Sb, e.g., which may be GeSbTe, N doped GeSbTe, Sidoped GeSbTe, InGeSbTe, or stoichiometric materials. In someembodiments, the first phase change material 211 may includesuperlattice structure. In some embodiments, the second phase changematerial 211′ may include superlattice structure. In some embodiments,the first phase change material 211 has the same material with thesecond phase change material 211′. In some other embodiments, the firstphase change material 211 has different material with the second phasechange material 211′. As previously discussed in FIG. 2A, a thicknesst211 of the first phase change material 211 is in a range of from 20Angstrom to 500 Angstrom. In some embodiments, the second phase changematerial 211′ has a thickness in a range similar to the first phasechange material 211. In some embodiments the thickness t211′ of thesecond phase change material 211′ is different from the thickness t211of the first phase change material 211. While in some embodiments, thethickness t211 is identical with the thickness t211′.

In some embodiments, the first phase change material 211 and the secondphase change material 211′ can switch between distinct metastable phasesby altering Joule heat provided by the bottom electrode 101 and/or thetop electrode 104. As previously discussed in FIG. 2A, the top electrode104 and/or the bottom electrode 101 is selected to possess a suitablethermal conductivity, or is designed to possess heat-retentionstructures, in order to effectively achieve the phase changetemperature.

In some embodiments, the phase change memory structure illustrated inFIG. 3A may further include one first middle electrode 300 between thetop electrode 104 and the bottom electrode 101, as shown in FIG. 3B. Aspreviously discussed in FIG. 2B, compared to the top electrode 104 andthe bottom electrode 101, the first middle electrode 300 is selected tohave a different thermal property, for example, a lower heat-retention,from its top and bottom counterparts due to the fact that one end of themiddle electrode 300 is in contact with the first switch 111, which doesnot require elevated temperature as the first or the second changematerial 211/211′ does. Referring to FIG. 3B, a cross section of a phasechange memory structure, the first middle electrode 300 may be disposedbetween the first switch 111 and the second phase change material 211′.The first middle electrode 300 may also be spacing between otherswitch-phase change material interfaces, for example, between the firstswitch 111 and the first phase change material 211, or between thesecond phase change material 211′ and the second switch 111′. Aspreviously discussed in FIG. 2B, in some embodiments, a thickness t300of the first middle electrode 300 is in a range of from 20 Angstrom to500 Angstrom.

Referring to FIG. 3C, the phase change memory structure illustrated inFIG. 3A may further include multiple middle electrodes between the topelectrode 104 and the bottom electrode 101. For example, the firstmiddle electrode 300 is spacing between the first phase change material211 and the first switch 111; a second middle electrode 300′ is disposedbetween the first switch 111 and the second phase change material 211′;while a third middle electrode 300″ is disposed between the secondswitch 111′ and the second phase change material 211′. In someembodiments, a thickness t300′ of the second middle electrode 300′ is ina range similar to the thickness t300 of the first middle electrode 300.In some embodiments, a thickness t300″ of the third middle electrode300″ is in a range similar to the thickness t300 of the first middleelectrode 300.

Referring to FIG. 4A, FIG. 4A is a cross section of a phase changememory structure. The phase change memory structure may include thephase change memory stack 140 disposed between the N^(th) ILD 128 andthe (N+1)^(th) ILD 128′. The phase change memory stack 140 is surroundedby the dielectric layer 127. In some embodiments, the phase changememory stack 140 includes the bottom electrode 101 disposed above theN^(th) ILD 128, the first switch 111 contacting a top surface of thebottom electrode 101, the first phase change material 211 disposed abovea top surface of the first switch 111, the second switch 111′ disposedabove a top surface of the first phase change material 211, and the topelectrode 104 disposed between a top surface of the second switch 111′and the bottom surface of the (N+1)^(th) ILD 128′. In some embodiments,the first switch 111 and the second switch 111′ have high resistanceunder voltages below thresholds voltage value of the first switch 111and the second switch 111′ respectively. If an applied voltage exceedsthe threshold voltage value, the resistances of the switches becomesignificantly lower. As previously discussed in FIG. 2A, in someembodiments, a thickness t111 of the first switch 111 is in a range offrom 20 Angstrom to 500 Angstrom. A thickness t111′ of the second switch111′ is in a range similar to the thickness t111 of the first switch111.

In some embodiments, the first switch 111 and the second switch 111′ mayinclude commonly used chalcogenide materials including, but not limitedto, one or more of As, Ge, and Se, e.g., which may be AsGeSe, N dopedAsGeSe, Si doped AsGeSe, InAsGeSe, or stoichiometric materials. In someembodiments, the first switch 111 may include superlattice structure. Insome embodiments, the second switch 111′ may include superlatticestructure. In some embodiments, the first switch 111 is identical to thesecond switch 111′. In some other embodiments, the first switch 111 isdifferent from the second switch 111′. Conventionally, only one switchis disposed in one phase change memory structure. Along the path ofthreshold voltage reduction, thickness of the switch has to be madethinner to an extent that triggers substantial leakage current.Therefore, directly reducing the thickness of the switch to lower thethreshold voltage is at the expense of increasing the leakage current.By having at least two switches arranged in series in one phase changememory structure, the properties of the phase change memory structure,such as leakage current or threshold voltage, can be easily tuned bytailoring various kinds of materials and thicknesses of switches. Aspreviously discussed in FIG. 2A, in some embodiments, a thickness t101of the bottom electrode 101 is in a range of from 20 Angstrom to 500Angstrom. A thickness t104 of the bottom electrode 104 is in a rangesimilar to the thickness t101 of the bottom electrode 101.

Still referring to FIG. 4A, the first phase change material 211 has twodistinct metastable phases, e.g. crystalline and amorphous, related todifferent resistivity. In some embodiments, the first phase changematerial 211 may include commonly used chalcogenide materials including,but not limited to, one or more of Ge, Te, and Sb, e.g., which may beGeSbTe. N doped GeSbTe, Si doped GeSbTe. InGeSbTe, or stoichiometricmaterials. In some embodiments, the first phase change material 211 mayinclude superlattice structure. As previously discussed in FIG. 2A, insome embodiments, a thickness t211 of the first phase change material211 is in a range of from 20 Angstrom to 500 Angstrom.

In some embodiments, the first phase change material 211 can switchbetween distinct metastable phases by altering Joule heat provided bythe bottom electrode 101 and/or the top electrode 104. As previouslydiscussed in FIG. 2A, the top electrode 104 and/or the bottom electrode101 is selected to possess a suitable thermal conductivity, or isdesigned to possess heat-retention structures, in order to effectivelyachieve the phase change temperature.

In some embodiments, the phase change memory structure illustrated inFIG. 4A may further include one first middle electrode 300 between thetop electrode 104 and the bottom electrode 101, as shown in FIG. 4B. Aspreviously discussed in FIG. 2B, compared to the top electrode 104 andthe bottom electrode 101, the first middle electrode 300 is selected tohave a different thermal property, for example, a lower heat-retention,from its top and bottom counterparts due to the fact that one end of themiddle electrode 300 is in contact with the second switch 111′, whichdoes not require elevated temperature as the first or the second changematerial 211/211′ does. Referring to FIG. 4B, a cross section of a phasechange memory structure, the first middle electrode 300 may be disposedbetween the second switch 111′ and the first phase change material 211.The first middle electrode 300 may also be spacing between otherswitch-phase change material interfaces, for example, between the firstphase change material 211 and the first switch 111. As previouslydiscussed in FIG. 2B, in some embodiments, a thickness t300 of the firstmiddle electrode 300 is in a range of from 20 Angstrom to 500 Angstrom.

Referring to FIG. 4C, the phase change memory structure illustrated inFIG. 4A may further include multiple middle electrodes between the topelectrode 104 and the bottom electrode 101. For example, the firstmiddle electrode 300 is spacing between the first phase change material211 and the first switch 111; while a second middle electrode 300′ isdisposed between the second switch 111′ and the first phase changematerial 211. In some embodiments, a thickness t300′ of the secondmiddle electrode 300′ is in a range similar to the thickness t300 of thefirst middle electrode 300.

Referring to FIG. 5A, FIG. 5A is a cross section of a phase changememory structure. The phase change memory structure may include thephase change memory stack 140 disposed between the N^(th) ILD 128 andthe (N+1)^(th) ILD 128′. The phase change memory stack 140 is surroundedby the dielectric layer 127. In some embodiments, the phase changememory stack 140 includes the bottom electrode 101 disposed above theN^(th) ILD 128, the first switch 111 contacting a top surface of thebottom electrode 101, the first phase change material 211 disposed abovea top surface of the first switch 111, the second switch 111′ disposedabove a top surface of the first phase change material 211, a secondphase change material 211′ disposed above a top surface of the secondswitch 111′, and the top electrode 104 disposed between a top surface ofthe second phase change material 211′ and the bottom surface of the(N+1)^(th) ILD 128′. In some embodiments, the phase change memory stack140 may further include one or more phase change materials and/or one ormore switches between the top electrode 104 and the second switch 111′.

As previously discussed in FIG. 2A, having two phase change materialsarranged in series could greatly reduce failure rate of the phase changememory structure. Referring to FIG. 5A, both the materials of the firstphase change material 211 and the second phase change material 211′ havetwo distinct metastable phases, e.g. crystalline and amorphous, relatedto different resistivity. In some embodiments, the first phase changematerial 211 and the second phase change material 211′ may includecommonly used chalcogenide materials including, but not limited to, oneor more of Ge, Te, and Sb, e.g., which may be GeSbTe, N doped GeSbTe, Sidoped GeSbTe, InGeSbTe, or stoichiometric materials. In someembodiments, the first phase change material 211 may includesuperlattice structure. In some embodiments, the second phase changematerial 211′ may include superlattice structure. In some embodiments,the first phase change material 211 has the same material with thesecond phase change material 211′. In some other embodiments, the firstphase change material 211 has different material with the second phasechange material 211′. As previously discussed in FIG. 2A, in someembodiments, a thickness t211 of the first phase change material 211 isin a range of from 20 Angstrom to 500 Angstrom. In some embodiments, thesecond phase change material 211′ has a thickness in a range similar tothe first phase change material 211.

In some embodiments, the first phase change material 211 and the secondphase change material 211′ can switch between distinct metastable phasesby altering Joule heat provided by the bottom electrode 101 and/or thetop electrode 104. As previously discussed in FIG. 2A, the top electrode104 and/or the bottom electrode 101 is selected to possess a suitablethermal conductivity, or is designed to possess heat-retentionstructures, in order to effectively achieve the phase changetemperature.

In some embodiments, the phase change memory structure illustrated inFIG. 5A may further include one first middle electrode 300 between thetop electrode 104 and the bottom electrode 101, as shown in FIG. 5B. Aspreviously discussed in FIG. 2B, compared to the top electrode 104 andthe bottom electrode 101, the first middle electrode 300 is selected tohave a different thermal property, for example, a lower heat-retention,from its top and bottom counterparts due to the fact that one end of themiddle electrode 300 is in contact with the second switch 111′, whichdoes not require elevated temperature as the first or the second changematerial 211/211′ does. Referring to FIG. 5B, a cross section of a phasechange memory structure, the first middle electrode 300 may be disposedbetween the second switch 111′ and the first phase change material 211.The first middle electrode 300 may also be spacing between otherswitch-phase change material interfaces, for example, between the firstswitch 111 and the first phase change material 211, or between thesecond phase change material 211′ and the second switch 111′. Aspreviously discussed in FIG. 2B, in some embodiments, a thickness t300of the first middle electrode 300 is in a range of from 20 Angstrom to500 Angstrom.

Referring to FIG. 5C, the phase change memory structure illustrated inFIG. SA may further include multiple middle electrodes between the topelectrode 104 and the bottom electrode 101. For example, the firstmiddle electrode 300 is spacing between the first phase change material211 and the first switch 111; a second middle electrode 300′ is disposedbetween the second switch 111′ and the first phase change material 211;while a third middle electrode 300″ is disposed between the secondswitch 111′ and the second phase change material 211′. In someembodiments, a thickness t300′ of the second middle electrode 300′ is ina range similar to the thickness t300 of the first middle electrode 300.In some embodiments, a thickness t300″ of the third middle electrode300″ is in a range similar to the thickness t300 of the first middleelectrode 300.

Referring to FIG. 6A, FIG. 6A is a cross section of a phase changememory structure. The phase change memory structure may include thephase change memory stack 140 disposed between the N^(th) ILD 128 andthe (N+1)^(th) ILD 128′. The phase change memory stack 140 is surroundedby the dielectric layer 127. In some embodiments, the phase changememory stack 140 includes the bottom electrode 101 disposed above theN^(th) ILD 128, a switch superlattice structure 112 contacting a topsurface of the bottom electrode 101, a phase change material 212disposed above a top surface of the switch superlattice structure 112,and the top electrode 104 disposed between a top surface of the phasechange material 212 and the bottom surface of the (N+1)^(th) ILD 128′.As previously discussed in FIG. 2A, the top electrode 104 and/or thebottom electrode 101 is selected to possess a suitable thermalconductivity, or is designed to possess heat-retention structures, inorder to effectively achieve the phase change temperature. One cycle ofthe switch superlattice structure 112 includes a first switch 112 a anda second switch 112 b stacking over the first switch 112 a. In someembodiments, the switch superlattice structure 112 may include 1 to 20cycles. In some embodiments, the switch superlattice structure 112 mayinclude 8 to 10 cycles. In some embodiments, a material of the firstswitch 112 a is different from a material of the second switch 112 b inthe same superlattice cycle. While in some other embodiments, a materialof the first switch 112 a is identical to a material of the secondswitch 112 b but only with different dopants.

One cycle of the phase change material 212 includes a first phase changematerial 212 a and a second phase change material 212 b stacking overthe first phase change material 212 a. In some embodiments, the phasechange material 212 may include 1 to 20 cycles. In some embodiments, thephase change material 212 may include 8 to 10 cycles. In someembodiments, a material of the first phase change material 212 a isdifferent from a material of the second phase change material 212 b inthe same superlattice cycle. While in some other embodiments, a materialof the first phase change material 212 a is identical to a material ofthe second phase change material 212 b but only with different dopants.In some embodiments, a thickness t112 a of the first switch 112 a and athickness t112 b of the second switch 112 b are about 10 Angstrom. Insome embodiments, a thickness t212 a of the first phase change material212 a and a thickness t112 b of the second phase change material 212 bare about 10 Angstrom.

The phase change memory structure may further include a middle electrode600 spacing between the switch superlattice structure 112 and the phasechange material 212, as shown in FIG. 6B. As previously discussed inFIG. 2B, compared to the top electrode 104 and the bottom electrode 101,the first middle electrode 600 is selected to have a different thermalproperty, for example, a lower heat-retention, from its top and bottomcounterparts due to the fact that one end of the middle electrode 300 isin contact with the switch superlattice structure 112, which does notrequire elevated temperature as the first or the second change material211/211′ does. In some embodiments, a material of the first middleelectrode 600 may be similar to the first middle electrode 300previously discussed in FIG. 2B. In some embodiments, a thickness t600of the middle electrode 600 is in a range of from 20 Angstrom to 500Angstrom. The criticality of the thickness t600 of the first middleelectrode 600 is similar to the thickness t300 of the first middleelectrode 300, as previously discussed in FIG. 2B.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother operations and structures for carrying out the same purposesand/or achieving the same advantages of the embodiments introducedherein. Those skilled in the art should also realize that suchequivalent constructions do not depart from the spirit and scope of thepresent disclosure, and that they may make various changes,substitutions, and alterations herein without departing from the spiritand scope of the present disclosure.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

Some embodiments of the present disclosure provide a phase change memorystructure, including a bottom electrode, a first phase change materialcontacting a top surface of the bottom electrode, a first switch overthe first phase change material, a second phase change material over thefirst switch, and a top electrode over the second phase change material.

Some embodiments of the present disclosure provide a phase change memorystructure, including a bottom electrode, a first switch contacting a topsurface of the bottom electrode, a first phase change material over thefirst switch, a second switch over the first phase change material, atop electrode over the second switch.

Some embodiments of the present disclosure provide a phase change memorystructure, including a bottom electrode, a switch superlattice structurecontacting a top surface of the bottom electrode, the switchsuperlattice structure includes a first switch and a second switchstacking with the first switch wherein a material of the first switch isdifferent from a material of the second switch, a phase change materialover the switch superlattice structure, and a top electrode over thephase change material.

What is claimed is:
 1. A phase change memory structure, comprising: abottom electrode; a first switch over the bottom electrode; a secondswitch over the first switch; a phase change material spacing betweenthe first switch and the second switch; a top electrode over the secondswitch.
 2. The phase change memory structure of claim 1, wherein a firstthreshold voltage of the first switch is different from a secondthreshold voltage of the second switch.
 3. The phase change memorystructure of claim 1, wherein the second switch is doped with silicon.4. The phase change memory structure of claim 1, wherein the secondswitch comprises one of AsGeSe, N doped AsGeSe, Si doped AsGeSe,InAsGeSe.
 5. The phase change memory structure of claim 1, wherein amaterial of the first switch is different from a material of the secondswitch.
 6. The phase change memory structure of claim 5, wherein one ofthe first switch and the second switch comprises superlattice structure.7. The phase change memory structure of claim 1, further comprising atleast one middle electrode between the top electrode and the bottomelectrode, wherein the middle electrode possesses a different heatconductivity from that of the top electrode.
 8. The phase change memorystructure of claim 7, wherein the middle electrode is between the firstswitch and the second switch.
 9. The phase change memory structure ofclaim 1, further comprising a dielectric layer surrounding a sidewall ofthe first switch and a sidewall of the second switch.
 10. A phase changememory structure, comprising: a bottom electrode; a first switchcontacting a top surface of the bottom electrode, wherein the firstswitch has a first thickness; a second switch over the first switch,wherein the second switch has a second thickness different from thefirst thickness; a phase change material spacing between the firstswitch and the second switch; and a top electrode over the secondswitch.
 11. The phase change memory structure of claim 10, wherein amaterial of the first switch is identical with a material of the secondswitch.
 12. The phase change memory structure of claim 10, wherein thesecond switch is doped with silicon.
 13. The phase change memorystructure of claim 10, further comprising a middle electrode between thefirst switch and the second switch.
 14. The phase change memorystructure of claim 10, wherein the first thickness is in a range from 20Angstrom to 500 Angstrom.
 15. The phase change memory structure of claim10, wherein at least one of the first switch and the second switchcomprises superlattice structure.
 16. The phase change memory structureof claim 10, wherein a first threshold voltage of the first switch isdifferent from a second threshold voltage of the second switch.
 17. Aphase change memory structure, comprising: a bottom electrode; a firstswitch contacting a top surface of the bottom electrode, wherein thefirst switch is doped with a first dopant; a second switch over thefirst switch, wherein the second switch is doped with a second dopantdifferent from the first dopant; a phase change material spacing betweenthe first switch and the second switch; and a top electrode over thesecond switch.
 18. The phase change memory structure of claim 17,wherein the first switch comprises AsGeSe.
 19. The phase change memorystructure of claim 17, wherein the phase change material comprises oneof GeSbTe, N doped GeSbTe, Si doped GeSbTe, or InGeSbTe.
 20. The phasechange memory structure of claim 17, wherein the first dopant is N orSi.